Ignore ConstantExpr in IgnoreParens

Summary:
This moves it up from IgnoreParenImpCasts to IgnoreParens, so that more
helpers ignore it. For most clients, this ensures that these helpers
behave the same with and without C++17 enabled, which is what appears to
introduce these new expression nodes.

Fixes PR39881

Reviewers: void, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55853

llvm-svn: 350068